VLBI observations of the maser emission in the four ground-state OH transitions from W3/OH/ and SGR B2

Abstract

We have obtained accurate, relative positions of the maser emission in all four ground-state OH transitions from W3(OH) with VLBI observations. All ground-state OH maser emission comes from the vicinity of the compact H ii region; however, the strongest maser features at 1612 and 1667 MHz do not coincide with any clusters of the very intense 1665-MHz emission. These results suggest that the different OH transitions are pumped under similar, but not identical, physical conditions. A comparison of total-power spectra taken over nearly a 15-yr time span shows some changes, but suggests that most OH maser features have lifetimes greater than several decades. The four ground-state OH masers of Sgr B2 were totally resolved and have apparent sizes greater than 0.05 arcsec. Such large sizes are probably a result of interstellar plasma scattering.